For some time, shot peening has been applied in order to improve fatigue strength by imparting compressive residual stress to steel products such as automobile parts, dies, and the like.
In this type of shot peening, a two stage peening is performed, whereby shot peening is carried out under relatively high strength peening conditions using a large projection material, then shot peening is performed under lower strength peening conditions to optimize the distribution of residual stress.
For example, Patent Document 1 discloses a technology for changing the hardness and particle size of a heat treated mold (steel product) by appropriately changing the hardness and particle size of spherical projection material (shot) and performing multiple peening iterations.